This invention relates generally to lasers of the type generating linearly polarized light and particularly relates to a polarized laser which is self-aligned.
It is well known that lasers have to be precisely aligned for their proper operation. If the output reflector of the laser becomes misaligned the cavity loss may increase to the point that the laser system is no longer capable of lasing. For example, a ruby laser is generally aligned with an auto collimator disposed in the cavity. The collimator may be permanently included in the laser or may be temporarily inserted. The alignment of the output reflector accomplished by means of the auto collimator must be checked once a day or even more frequently in many cases. Great rigidity and alignment precision is also required of those gas lasers utilizing Brewster windows and which, of course, generate linearly polarized light.
The patent to Kantorski et al. U.S. Pat. No. 3,670,263 relates to a laser structure which is self-aligning due to its particular mechanical construction. This patent explains in some detail the problems due to misalignment of a laser.
In accordance with the present invention as indicated above, a corner reflector is utilized. Such a corner reflector may either be a transparent tetrahydral structure or it may consist of three reflectors disposed normal or at right angles to each other. A corner reflector takes its name from the fact that it may be visualized as the corner of a cube.
Two angular directions are required to specify the direction of a light beam, say, for example, altitude and azimuth. A corner reflector returns a beam exactly along the direction of its incidence to the corner reflector. Its direction of propagation is reversed but the two direction angles are unchanged. Alternately these same retro reflections may be described by saying that both angular propagation directions are increased by 180.degree..
In contrast to the corner reflector, the roof reflector does not return a beam with both direction angles unchanged. By proper choice of a coordinate system, the roof reflector may be seen to return the beam with one angle unchanged but the other angle will be changed unless it was rigourously perpendicular to the roof angle. The net consequence of this for the present application is that the corner reflector returns the incoming beam exactly in the same direction it was incident independent of the angular orientation of the corner reflector. The return from the roof reflector on the other hand changes the return beam angle relative to the input beam as the roof reflector is moved.
It should be noted that to completely describe the return of a light ray not only its propagation direction angles need to be defined but also its translational position. For the corner reflector a ray entering at one side of the assembly is returned from the other side (from the point symmetric with respect to the corner vertex), the direction of the ray being as described above. Thus the ray returns in the same angular direction but offset to one side.
Among patents showing a roof reflector in connection with the laser, reference is made to the patent to Stickley U.S. Pat. No. 3,327,243, the patent to Miller U.S. Pat. No. 3,414,835 and to Woodbury et al. U.S. Pat. No. 3,464,026.
It has also been proposed to provide a corner reflector in connection with a laser. However, the patent to Boyden et al. U.S. Pat. No. 3,397,024 shows a corner reflector outside of the laser cavity.
In accordance with the patent to Eckberg U.S. Pat. No. 3,447,098, a corner reflector is used to reflect the laser beam from one face of a gas laser, the beam from the other face being reflected by a refractor or mirror. However, the operation of this laser is not entirely clear. No mention is made of the fact that the corner reflector will generate two parallel laser beams. In addition, a helium-neon laser must be of rather small diameter in order to lase. In other words, the diameter of the tube cannot exceed 2 or 3 millimeters because the wall effect is essential to the lasing operation. No consideration has been given in this patent to the importance of the direction of polarization.
Finally, the patent to Bullinger U.S. Pat. No. 3,617,926 discloses a gas laser having an optical resonator cavity formed by three reflective means. One of these is a corner reflector, while the other two reflectors will reflect the two parallel laser beams. The light generated by the gas laser is polarized due to the two Brewster windows. Two additional Brewster windows are associated with the corner reflector for a purpose which is not made evident. Unless the corner reflector is carefully aligned or oriented with respect to the plane of polarization of the light generated by the laser, the device will not properly function. Thus, if the corner reflector is a transparent solid body, it will modify the polarization so that the Brewster windows will not work as planned. If the corner reflector consists of three metallic mirrors disposed at right angles to each other in an attempt to achieve a return polarization the same as the incident polarization, it will generate too much loss for a helium-neon laser.
It is accordingly an object of the present invention to provide a self-aligned polarized laser which is capable of withstanding a substantial amount of misalignment without ceasing to lase.
A further object of the present invention is to provide a laser which generates linearly polarized light and which includes a corner reflector where careful consideration is given to the direction of polarization of the light which is returned by the corner reflector.